THE 

CAUSE OF RUSTINESS 



BY 

T. EGLESTON, Ph.D, 

» » 

SCHOOL OF MINES, NEW YORK CITY. 


A Paper Read before the American Institute of Mining 
Engineers, at the Philadelphia Meeting, February, 1881. 

•j > 


AUTHOR’S EDITION. 
18 8 1 . 

































[TRANSACTIONS OF THE AMERICAN INSTITUTE OF MINING ENGINEERS.] 






3 




THE CAUSE OF RUSTINESS AND OF SOME OF THE LOSSES 

IN WORKING GOLD. 

BY T. EGLESTON, PII.D., SCHOOL OF MINES, NEW YORK CITY. 

There has always been a theory among those working placer 
mines that gold is both found a rusty,” anil becomes so under treat¬ 
ment, by which they mean, not that gold becomes coated with oxide 
of gold, but that it is either coated superficially or alloyed with some 
substance which prevents the contact with mercury, and thus pre¬ 
cludes the possibility of amalgamation. To such conditions of the 
gold are ascribed the losses in working most placer deposits." Rusty 
gold is often talked about, but is not often shown, and when pro¬ 
duced oftener amalgamates than not. It is frequently covered with 
a brownish coating, and has a much redder color than ordinary gold. 
The coating is very irregularly distributed over the surface, every 
spot where the least abrasion has occurred showing the true gold- 
yellow. All such spots form contact surfaces, and cause the whole 
piece to amalgamate readily. When the gold is wholly coated it 
resists the action of the mercury for a considerable time and perhaps 
altogether. This coating on gold is due to the superficial action of 
some substance, which is soluble and is often precipitated at the 
same time and by the same causes as the gold, for fine particles of gold 
are sometimes visible with the microscope in the detached coating. 
This coating has never to my knowledge been carefully analyzed. 
Those who describe it say that it often cracks off from pieces of gold 
leaving them bright. Nuggets of some size are said to have been 
found in South America, in alluvial soil, coated with a silicate of 
iron containing considerable gold. Very often the film is entirely 
composed of silica, which is deposited on and beside the gold. This 
silica is sometimes opaque, and again quite transparent, so that 
the gold can be seen disseminated through it with the microscope, 
just as cinnabar crystals are seen in the red chalcedony of the dis¬ 
trict around Knoxville, Cal. When the silica surrounds the gold 
entirely it prevents the gold from being attacked by the mercury, 
just as the fine particles of gold from the veins are carried off in the 
rock when it has not been rendered sufficiently fine in the crushing. 

1 


^3 - V 77 7 


2 CAUSE OF RUSTINESS AND LOSSES IN WORKING GOLD. 


While there are many artificial causes which produce the rustiness 
of gold, this covering of the surface with particles of some foreign 
substance is probably the only one which occurs in nature. In the 
separation of gold from its ore there are a number of causes which 
render it rusty or prevent its amalgamation in the mill. Some of 
these causes I have recently had occasion to investigate, and are, I 
believe, now announced for the first time; others have been more or 
less known for a number of years. 

It has been asserted that the presence of certain substances alloyed 
with the gold would prevent its amalgamation. This I have gen¬ 
erally found not to be the case. I have made a number of these 
alloys and have found them to amalgamate without difficulty, but 
chemical combinations frequently do resist the action of the mercury 
altogether, or when they yield, it is attended with the formation of 
a chemical compound with the mercury, and the consequent Toss of 
the latter. 

In order to ascertain the causes which prevent the amalgamation of 
gold I undertook to reproduce artificially the conditions which were 
supposed to prevent it. These I found to be mechanical and chemical. 
I found that if a piece of soft gold which could be easily bent, and 
amalgamated readily, was hammered on a perfectly bright anvil, 
with a bright hammer, giving several rapid blows, until the gold 
had become hard, and had acquired a certain amount of elasticity, it 
would remain in juxtaposition with mercury for a very long time 
without being affected by it. This hammering increases the density 
of the metal and closes the pores, so that [ have recently had a piece 
of gold which was put into this condition by hammering, remain 
nearly two weeks floating on mercury without being attacked. If 
the gold in this condition is heated and cooled slowly, it again 
amalgamates rapidly. If, however, it is cooled rapidly by plunging 
it suddenly into very cold water the amalgamation takes place very 
slowly. The same metal, after being repeatedly rapidly cooled and 
heated, amalgamates more readily than if it has only been treated 
once. 


To ascertain the effect of different substances on the gold, a strip 
of clean gold which readily amalgamated was dipped into a solu¬ 
tion of sulph-hydrate of ammonia, and also into one of sulphu¬ 
retted hydrogen.. After being withdrawn from these liquors and 
allowed to dry, mercury would not touch either of the pieces. The 
effects of grease are well known, and the greatest care is taken in 
most mills to keep the “ quick ” bright, either by rubbing or by the 


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CAUSE OF RUSTINESS AND LOSSES IN WORKING GOLD. 3 

addition of chemicals, to such a extent in some cases that the chemi¬ 
cals themselves used in excess are often a cause of the very thing 
they are intended to prevent. All these causes, the hammering, the 
effect of sulphuretted waters, and also the effect of grease, occur in 
every mill, and probably have something to do with the losses in 
gold which take place there. 

The effect of the different elements likely to be found with gold was 
also investigated. A strip of gold which would amalgamate easily 
was exposed to vapors of sulphur, after which it would not amalga¬ 
mate. When the piece was heated in the flame of a Bunsen burner, 
the film of sulphide was decomposed, and the gold amalgamated 
readily. To test the effects of arsenic and antimony upon gold, 
these metals, in variable proportions, were melted with it, and it 
was ascertained that considerable quantities of arsenic and antimony 
were in the alloys. Both these alloys amalgamate very easily. 
Gold and arsenic and gold and antimony were then precipitated 
together, and it was found that the precipitates were compounds of 
antimony and gold and of arsenic and gold; they also amalgamated 
readily. It has been asserted that .0004 gram of antimony to the 
ton of auriferous pyrites of Grass Valley, and .001 gram in that of 
Callao, was sufficient to prevent the amalgamation of the gold.* 
This seems, however, to be not quite exact, if the statement given 
of the conditions in which the gold is found be correct, for there 
does not seem to be any reason why an alloy with a much larger 
amount, as in the artificial compounds, should amalgamate readily, 
and the natural one containing so much less, refuse to do so. 

The effect of sulphur was further studied by making a regular sul¬ 
phide of gold. When this was heated it was found that the sulphur 
was readily driven off by ignition, and the residue, which was bright- 
yellow gold of exactly the same shape as the sulphide, easily amal¬ 
gamated. A phosphide of gold was then made by pouring melted 
phosphorus upon hot sponge-gold, and expelling the excess of phos¬ 
phorus by heating for some time in a non-oxidizing atmosphere. 
It did not amalgamate. Oxide of gold was made, but this com¬ 
pound is so very unstable and so readily reduced to metallic gold bv 
simple rubbing that it could not possibly have any effect on prevent¬ 
ing amalgamation, although it is not attacked by mercury. An 
amalgam of gold was then placed in acid, which easily dissolved out 
the mercury, and left behind a brown crystalline residue of gold, 
which readily took up mercury again. 


* Comptes Rendus, March 17th, 1879. 



4 CAUSE OF RIJSTINESS AND LOSSES IN WORKING GOLD. 

It will thus be seen that the stamp-mill is not a rationally designed 
machine. The action of pounding is likely to put some of the gold 
into such a condition that the mercury will not touch it, and to flour 
the gold, as well as the quicksilver. There is, besides, in the mill every 
probability of the introduction of grease or greasy substances, like the 
powdered hydrated silicates of magnesia and of alumina, which not 
only froth but coat the gold with a slime which prevents the action of 
the mercury. If the water used in the mill is not pure, there is a fur¬ 
ther likelihood of the introduction of sulphuretted hydrogen, and of 
other soluble sulphides, which act superficially on the small particles 
of gold, and prevent the action of the mercury. I have elsewhere 
pointed out some of the other causes of losses in the treatment of gold.* 
The pounding action of the stamp has doubtless the effect to make 
some of the particles of gold wholly unattackable by mercury, while 
the action of rubbing that occurs in the arastra is much more likely 
to pulverize the fine particles of pyrites, to break up any coating 
that may be around the particles of gold, and to rub off any super¬ 
ficial deposit, and thus bring the gold into contact with the mercury 
and make it amalgamate. It would be folly to advocate the rein¬ 
troduction of the arastra which is so limited in its output, but I am 
satisfied that we shall have to come back to its principle. It is a 
very remarkable fact that when in the early days Mexicans with the 
arastra got $50 to $60 a day, the stamp-mill working on the same 
rock obtained only $15 to $20,.and instances are cited where with 
the best modern machinery only $20 to $30 can be got out of rock 
which yields $700 to $800 by the fire assay. One or two machines 
have recently been constructed on the arastra principle, but no sys¬ 
tematic tests have been made of them and little done beyond the 

* 

singing of their praises by enthusiastic inventors. 

* On the Formation of Gold Nuggets and Placer Deposits. A paper read at 
the Philadelphia Meeting, February, 1881. 






